I. Field of the Invention
This invention relates to shock dampening devices for use in drill strings and, in particular, to an apparatus designed to control the force applied to the drill bit in both vertical and deviated holes by absorbing and decoupling axial vibrations and torsional forces acting upon the drill string.
II. Description of the Prior Art
Severe axial and torsional forces induced into the drilling assembly during drilling operations can cause damage and wear on the components of the drill string including the drilling tool and the various measuring devices. Such forces can be found in both conventional vertical drilling and high angle drilling where the position of the drill bit is critical. In addition, various conditions arise during drilling operations which induce a torsional or axial load into the drilling assembly. Hard rock and sticky earth formations can cause severe axial and torsional forces to be induced to the well string. The use of drag bits or roller cone bits can induce axial vibrations in vertical drilling operations. The frictional forces between the drill pipe and the hole in deviated holes can induce torsional forces thereby making it difficult to determine and control the position of the bit. In each of these cases, the unexpected release of these forces in the drill string can cause the down hole assembly to be slammed against the bottom of the hole.
Various devices have been developed which dampen or absorb the vertical or axial shocks applied to the drill string through the drill bit. Such shock absorbing subs may employ mechanical springs, resilient washers or fluid chambers to dampen or limit relative movement between an inner mandrel and an outer housing. Typically, the outer housing is connected to the bottom hole assembly. Rotational torque may be transmitted from the upper string to the bit by a series of longitudinal splines connecting the housing to the mandrel. While past known shock subs are capable of dissipating small shocks or loads of very short duration and greater magnitude, such devices are not totally satisfactory in absorbing the axial and torsional forces encountered by the well string. In addition, such tools provide no means of controlling the weight on bit in order to precisely determine the position of the bit relative to the bottom of the hole.
In addition to vertical shock loads, it is known that drill strings are subject to torsional forces resulting from rotation of the string. Such forces may result from sudden stopping of the rotary drill string due to bit hang up or over a longer period as a result of friction in deviated holes or in motors utilized with coiled tubing. Attempts have been made to dissipate such radial shock loads by translating such loads to a vertical component which is absorbed by the shock sub assembly of the device. Thus, both radial and vertical shocks must be dissipated by the same assembly which may overwhelm the tool and result in failure, causing damage to the drill string. Moreover, such tools do not provide means for controlling the weight on bit in order to position the bit in the hole.